The present invention relates generally to radiotelephones and more particularly to the coherent demodulation of differentially encoded quadrature phase shift keying (DQPSK) signals.
The performance of receivers in wireless radio communications systems may degrade severely due to multipath fading. Although anti-fading techniques, like antenna diversity, equalization, and adaptive array processing, may be very effective in improving the performance of the receiver, forward error correction (FEC) techniques may be necessary to achieve acceptable voice and data transmission in wireless communication systems. FEC techniques provide redundancy by adding extra bits to the actual information bits, which allows the decoder to detect and correct errors. In the receiver, the decoding process can be performed by either using hard information values or soft information values, which are provided by the demodulator. Decoding using soft information values improves the receiver performance over decoding using hard information values. Therefore, to improve decoder performance, it may be important to provide accurate soft information from the demodulation process.
The U.S. digital cellular system (IS-136) uses xcfx80/4 shifted-DQPSK as a modulation scheme. Differential encoding of the transmitted signals allows both coherent and differential demodulation of the received signal. Although differential demodulators may not be complex to implement, it is widely accepted that the performance of differential detectors degrades rapidly in the presence of Inter-Symbol Interference (ISI) which may be caused by multi-path propagation. Therefore, coherent demodulators with equalizers are commonly used in many receivers. Such a receiver is described in U.S. Pat. No. 5,285,480 to Chennakeshu et al., entitled ADAPTIVE MSLE-VA RECEIVER FOR DIGITAL CELLULAR RADIO.
FIG. 1 depicts a block diagram of a xcfx80/4 shifted-DQPSK system with a conventional differential demodulator receiver. The transmitter 105 includes encoder 101 and differential modulator 102. Information bits are encoded in encoder 101 to produce coded bits. The coded bits are differentially modulated in differential modulator 102 to produce a differentially modulated signal, which is provided to antenna 104 for transmission. The transmitted signal reaches the radio receiver after passing through a propagation medium (e.g., a mobile radio channel). The transmitted signal plus any noise are received at the receiver antenna 106 and the received signal provided to receiver 114. The received signal is processed by the radio processor 108 which amplifies, mixes, filters, samples and quantizes the received signal to produce a baseband signal. A differential demodulator 110 demodulates the received signal and provides symbol or bit values to the decoder 112 which decodes the encoded bits and which may detect and correct possible errors in the received signal. As discussed above, the output of the demodulator 110 is preferably soft values which may provide higher performance in decoding.
Differential encoding of the transmitted signals allows both coherent and differential demodulation of the received signal. FIG. 2 shows a block diagram of a known apparatus for differential demodulation of the DQPSK modulated signals. The differential detector uses received samples to get hard and/or soft decision values. The present received sample is coupled to the multiplier 203. The present received sample is also fed into a delay 201. The delay 201 is coupled to a conjugate operator 202, and the output of the conjugate operator 202 is coupled to the multiplier 203.
In operation, the present received sample and the delayed and conjugated received sample are multiplied to undo the effect of the differential encoder at the transmitter. The real 204 and imaginary 205 part of the output of the multiplier provide the soft bit values corresponding to the two bits sent in one, di-bit symbol. Also, the hard values can be obtained by taking the sign 206 and 207 of the soft values as desired.
FIG. 3 shows a block diagram of a known apparatus for coherent demodulation of the DQPSK modulated signals. The coherent receiver utilizes channel estimation unit 302 which estimates the amplitude and phase of the mobile radio channel. These channel estimates are passed to the coherent QPSK demodulator 301 where the estimates of the QPSK symbols are generated. The channel parameters can be estimated using the known data sequences which are periodically inserted into the transmitted information sequences. In systems where the channel parameters change over the transmission of two consecutive known data sequences, like the U.S. digital cellular system (IS-136), it is desirable to adapt the channel parameters during the transmission of unknown data sequences. Such an adaptive coherent receiver is described in U.S. Pat. No. 5,285,480 to Chennakeshu et al. entitled ADAPTIVE MLSE-VA RECEIVER FOR DIGITAL CELLULAR RADIO.
The output values of the coherent QPSK demodulator 301, which are the hard coherent symbols, are passed through a differential detector 303 to undo the effect of the differential encoder in the transmitter. The outputs of the differential detector are the hard decision values corresponding to the transmitted information bits.
A semi-coherent demodulation of the DQPSK modulated signals is described in U.S. Pat. No. 5,706,313 to Blasiak et al. entitled SOFT DECISION DIGITAL COMMUNICATION AND METHOD AND APPARATUS in which only the phase and frequency offset are estimated using the received signal. After compensating the phase and frequency offset, the likelihood of each possible QPSK symbol value for each sample is calculated. Therefore, a likelihood vector for each sample is obtained and this likelihood vector is provided to the decoder. The decoder uses the likelihood vectors to estimate the transmitted symbol values.
Another conventional method for the soft decoding of differentially encoded QPSK signal is described in U.S. Pat. No. 5,754,600 to Rahnema entitled METHOD AND APPARATUS FOR OPTIMUM SOFT-DECISION VITERBI DECODING OF CONVOLUTIONAL DIFFERENTIAL ENCODED QPSK DATA IN COHERENT DETECTION. In this apparatus the differential and Viterbi decoders are integrated, i.e., differential decoding is part of the convolutional decoding process.
Soft information for maximum likelihood sequence estimation (MLSE) for frequency selective fading channels has been extensively studied, for example as described in J. Hagenauer and P. Hoeher, xe2x80x9cA Viterbi algorithm with soft-decision outputs and its applicationsxe2x80x9d, Proceeding of IEEE Globecom Conference, pp. 47.1.1-47.1.7, Dallas, Tex., USA, November 1989. These techniques have been extended to xcfx80/4 shifted-DQPSK systems for example as described in Jong Park, Stephan B. Wicker and Henry L. Owen, xe2x80x9cSoft Output Equalization Techniques for xcfx80/4 DQPSK Mobile Radioxe2x80x9d, IEEE International Conference on Communications, pp. 1503-1507, Dallas, Tex., USA, 1997. However, relatively little work has been directed towards soft information generation for coherent detection of xcfx80/4 shifted-DQPSK in non-ISI channels, i.e. channels without significant inter-symbol interference (ISI). A suboptimal approach is given that requires exponentiation and logarithm operation in Yow-Jong Liu, Mark Wallace and John W. Ketchum, xe2x80x9cA Soft-Output Bidirectional Decision Feedback Equalization Technique for TDMA Cellular Radioxe2x80x9d, IEEE Journal on selected areas in commun., Vol. 11, No. 7, September 1993. This approach does not consider all possible symbol values in determining a soft value and, further, does not result in a unique solution.
In light of the above discussion, a need exists for improved performance in soft value determination for coherent demodulation of differentially encoded signals for non-ISI channels.
In view of the above discussion, it is an object of the present invention to provide accurate soft values for differentially encoded information.
A further object of the present invention is to provide soft values in a manner which does not require complex implementation.
Still another object of the present invention is to provide soft values from coherent detection of differentially encoded bits or symbols.
These and other objects of the present invention are provided by methods and systems which generate soft values from signal samples of a differentially encoded signal by estimating channel coefficients associated with the signal and determining metrics for each possible combination of current and previous coherent symbol values which indicate the probability of a possible differential bit value being encoded in the signal using the estimated channel coefficients and the signal samples. Soft values associated with a bit or symbol encoded in the signal are then generated based on the determined metrics.
The use of each potential current and previous coherent symbol value in determining a combined metric provides metrics which take into account the probability of each potential symbol value. Thus, by utilizing metrics associated with each potential symbol value, an accurate soft value may be obtained.
In a further embodiment of the present invention, determining a soft value is done by summing the metrics for each possible current and previous coherent symbol value that corresponds to a first differential bit value to provide a first value metric sum and summing the metrics for each possible current and previous symbol value corresponding to a second differential bit value to provide a second value metric sum. The first value metric sum may be divided by the second value metric sum to provide a ratio of probabilities of the differential bit value being the first value and differential bit value being the second value. Further, the metrics may be exponentiated and the exponentiated metrics summed for each possible symbol pair corresponding to a first bit value to provide a first value metric sum. The exponentiated metrics are also summed for each possible symbol pair corresponding to a second bit value to provide a second value metric sum. Also, the logarithm of the ratio of probabilities may be taken to provide the soft value for the bit.
Furthermore, where the differentially encoded signal is received at a plurality of antennas and channel coefficients associated with each of the plurality of antennas are estimated, metrics may be determined for each possible coherent symbol value which indicate the probability of a possible symbol value being encoded in the signal using the estimated channel coefficients and the signal for each antenna. In such a case, each metric is a sum of metrics for each antenna. Also, an impairment correlation may be determined for the plurality of antennas. Metrics may then be determined for each possible symbol value which indicate the probability of a possible coherent symbol value being encoded in the signal using the estimated channel coefficients, the impairment correlation and the signal for each antenna. A noise power associated with the received signal may be determined and the metrics determined based in part on the noise power.
In a particular embodiment, a detected value is determined from the signal samples and the channel coefficients. The detected values may be bit values for symbols encoded in the differentially encoded signal. Furthermore, the determined soft values may be associated the detected bit values.
Preferably, the signal samples are generated by a radiotelephone receiving and processing a differentially encoded signal.
In another embodiment of the present invention, soft values are generated from signal samples by estimating channel coefficients associated with the received signal and determining metrics for each possible coherent symbol value which indicate the probability of a possible symbol value being encoded in the signal samples of the signal using the estimated channel coefficients and the signal samples of the received signal. The maximum of the combined metrics for each possible current and previous symbol value corresponding to first bit value is determined to provide a first maximum metric value. The maximum metric of the combined metrics for each possible current and previous symbol value corresponding to a second bit value is also determined to provide a second minimum metric value. The second maximum metric value is then subtracted from the first maximum metric value to provide a difference of log likelihoods of the bit value being the first bit value and the bit value being the second bit value. By selecting the maximum values the complexity of the soft value determination may be reduced as the exponentiation and logarithm need not be determined.
In still another embodiment of the present invention, soft values are generated from signal samples by estimating channel coefficients associated with the signal and determining metrics for detected coherent symbol values which indicate the probability of the possible symbol values being encoded in the signal using the estimated channel coefficients and the signal samples of the signal. A detected term is determined from the metrics associated with the detected symbols which provides a first value metric corresponding to the detected differential bit. A second value metric, which corresponds to the non-detected bit, is obtained by obtaining the two different metric values by either flipping the current detected coherent symbol value or the previous detected coherent symbol value and keeping the other coherent symbol value as the detected value and selecting the maximum metric value among these two values. This alternative embodiment may further reduce the complexity of the soft value determination by basing the soft value determination on a detected symbol value.